Brad Marchand scored twice, Nathan Horton had a goal and an assist and Dennis Seidenberg also scored for the Bruins, who got 32 saves by Tuukka Rask. Their lineup included defenseman Tomas Kaberle, acquired by GM Peter Chiarelli earlier in the day, and center Chris Kelly, a former Senator who made his debut with the B's after being acquired on Tuesday.
"I got here 20 minutes before warmup, and sometimes it's the best way to do it," Kaberle said. "All the guys were great to me. It was a big win for us and it makes for an emotional day for me after 13 years in Toronto."
The Bruins also acquired center Randy Peverley and defenseman Boris Valabik from Atlanta in a deal that sent forward Blake Wheeler and defenseman Mark Stuart to the Thrashers. Peverley will join the Bruins on Sunday in Boston; Valabik will join the AHL Providence Bruins.
"I like the direction our management took and we hope to build on that," Bruins coach Claude Julien said.
Marchand scored his second goal of the game, Boston's third goal of the third period, to put the Bruins up 4-1 at 15:52, just 32 seconds after Seidenberg's power-play goal.
"We're the best team in the third period in the league and we really wanted to use it to our advantage," Marchand said. "We kept building as the game went on and we wanted another big third. These are big points for us and everyone brought it in the third."
Bobby Butler scored for the third straight game early in the second to give the Senators a brief 1-0 lead. Alex Kovalev scored a power-play goal with 1:30 left in the third. Robin Lehner stopped 29 shots for Ottawa, which swapped goalies with Colorado on Friday. GM Bryan Murray acquired Craig Anderson from the Avalanche in return for Brian Elliott after trading left wing Jarkko Ruutu to Anaheim on Thursday.
Anderson arrived during the first period, dressed and served as Lehner's backup.
"Luckily enough I didn't have to play with jet lag," Anderson said.
Anderson, who had struggled with the Avs after leading them to the playoffs last season, wasn't expecting to be moved.
"It was a big surprise," Anderson said. "Heading to the rink this morning, going for practice and ready for a little road trip to San Jose, and I got the phone call about two minutes from the rink saying that I've been moved. I didn't see it coming. I was just kind of surprised by it."
The Bruins showed that they think they can make a long playoff run by dealing former first-rounder Joe Colborne, their own first-round pick in June and another choice to the Leafs for Kaberle -- the kind of puck-moving defenseman they've been looking for.
"To have a guy like that it gives you extra confidence and obviously ability to make plays because you know that guys are going to have that puck," Bruins captain Zdeno Chara. "He's going to make that play, he's going to make that pass, and to have that it helps everybody, and everybody around is going to be better."
If Kaberle felt funny wearing No. 12 -- the No. 15 he wore in Toronto has long been retired in Boston in honor of Milt Schmidt -- imagine how Kelly felt making his Bruin debut against the only NHL team he'd ever played for.
"The first shift was a bit weird but once the game got going I finally felt better," said Kelly, who debuted with Ottawa in 2003-04. "The guys made it easy for me to come in here and play."
Marchand scored 6:47 into the second to tie it at 1, using Ottawa defenseman Andre Benoit as a screen to fire a wrist shot past Lehner into the top of the net.
Horton broke the tie with his 15th goal at 9:47 of the third. Seidenberg increased the lead to 3-1 with a power-play goal at 15:20.
Material from team media and wire services was used in this report